The present invention relates to a method and apparatus for determining slip thresholds for a propulsion slip control system to prevent the undesirable slipping of driven wheels of a motor vehicle, wherein the circumferential speed of each vehicle wheel is sensed and a vehicle speed and acceleration signal is derived therefrom along with slip signals for each driven vehicle wheel. The vehicle speed and acceleration signals are used to define a predetermined threshold value which is compared with the slip signals from each driven vehicle wheel for controlling the torque output of the motor vehicle.
German Offenlegungsschrift No. 3,545,652 discloses a device for controlling torque output in response to slip threshold value signals. In this device, a mean slip threshold is substituted by a lower value, when the vehicle travels faster than a predetermined speed in a curve and the vehicle longitudinal acceleration is within a specific range, for providing a propulsion slip control for the vehicle. Conversely, the means slip threshold is substituted by a higher value when the vehicle likewise travels in a curve and the vehicle longitudinal acceleration is above the above-mentioned range. An adaptation of the slip threshold to the stability required for travelling curves is therefore detectable in a sense when travelling curves. The coefficient of friction, i.e., the grip of the vehicle relative to the nature of the road just being traveled, is left out of consideration, so that a changeover is made to a lower slip threshold value in the case of dry gripping road, for example, although this is not actually necessary if the actual coefficient of friction of the road was considered.
A similar device is known from the non-prior published German Patent Application No. P 3,728,574.2, in which a raising of a slip threshold is performed when this slip threshold is exceeded by at least one driven wheel and the vehicle is simultaneously accelerating. This document says nothing as to the conditions for or the execution of a lowering of the slip threshold. In this device, and also partly in the one previously mentioned, a variation of the slip threshold is performed only when the prescribed slip threshold has already been exceeded and the propulsion slip control system is already in operation. It is therefore first determined if the prescribed slip threshold is obviously too low because the vehicle is still accelerating in spite of the operation of the propulsion slip control system, and only then is a prescribed higher slip threshold applied. This will only correspond to the optimum conditions in the rarest of cases, because it does not take into consideration the instantaneous grip of the road.
Constant slip thresholds which had to be matched to the least favorable case, i.e., smooth ice, have hitherto been used. However, these slip thresholds are too low for a higher coefficient of friction, so that only low accelerations are then possible and the propulsion slip control system is frequently switched on unnecessarily.
It is therefore the object of the present invention to provide a method which is capable of prescribing slip values which correspond to the instantaneous grip of the road and thus create optimum conditions as regards traction and stability for the briefest possible control action. By this thresholds are high, that is to say control only starts when the threshold is exceeded and is switched off again early. It is also an object of the invention to produce an apparatus for performing this method.
This objective obtained wherein the predetermined threshold value is defined by a specific curve plotting attainable acceleration from vehicle standstill (V.sub.F =0) as a function of the coefficient of friction (.mu.) determined for the respective type of vehicle and taking into consideration the resistances to motion which are a function of the vehicle speed (V.sub.F), and wherein the specific curve is extended to form a graph of the coefficient of friction (.mu.) as a function of the vehicle speed (V.sub.F) and of the vehicle acceleration (a.sub.x), and wherein a predetermined slip threshold value (.gamma..sub.s) is associated with each coefficient of friction (.mu.) determined from the graph.
It is possible by this method to determine the instantaneously effective coefficient of friction and to associate with the latter a slip threshold value which can serve as a guide quantity for the propulsion slip control system.
Further particulars will be found in the following description of the method according to the invention, and of an exemplary embodiment of an apparatus for performing this method. These and other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.